The attacker, who wielded an assault-type rifle and a handgun, opened fire inside Pulse, spraying clubbers with bullets. He was later killed in a shootout with Swat officers after taking hostages.

Authorities are investigating the attack - which left at least 53 other people in hospital, most of them critically ill - as an act of terrorism.

The suspected gunman was named as Omar Mateen, of Port St Lucie, Florida. The suspect's father recalled that his son recently got angry when he saw two men kissing in Miami and said this might be related to the assault.

One surgeon at Orlando Regional Medical Centre said the death toll was likely to climb.

All of the dead were killed with the assault rifle, according to US Representative Alan Grayson.

Witnesses described a chaotic scene when the gunfire began shortly before the club was due to close.

Jackie Smith, who saw two of her friends shot beside her, said: "Some guy walked in and started shooting everybody. He had an automatic rifle, so nobody stood a chance.

"I just tried to get out of there."

The suspect exchanged gunfire with a police officer working at the club, which had more than 300 people inside.

The gunman then went back inside and took hostages, Orlando police chief John Mina said.

At around 5am, a Swat team was sent in to rescue the hostages, culminating in the gunman's death.

Authorities are looking into whether the attack was an act of domestic or international terrorism, and if the shooter acted alone, according to Danny Banks, an agent with the Florida Department of Law Enforcement.

"This is an incident, as I see it, that we certainly classify as domestic terror incident," Orange County sheriff Jerry Demings said.

The previous deadliest mass shooting in the US was the 2007 attack at Virginia Tech, where a student massacred 32 people before killing himself.

The suspect's father, Mir Seddique, told NBC News about his son seeing two men kissing a couple of months ago.

"We are saying we are apologising for the whole incident," Mr Seddique said. "We are in shock like the whole country."

Mateen was said to have been known to the FBI before the nightclub attack and had been looked at by agents within the last few years.

The matter for which he came under investigation was "open and closed pretty quickly," an official said.

When asked if the gunman had a connection to radical Islamic terrorism, he said authorities had "suggestions that individual has leanings towards that".

Multiple news outlets are reporting that the gunman called 911 shortly before the attack and pledged allegiance to the Islamic State terror group.

The suspect had been licensed as a private security officer in Florida, with s tate records showing he had held a firearms licence since at least 2011. It was set to expire in September 2017.

Mateen's father said the attack had nothing to do with religion.

US president Barack Obama called the shooting an "act of terror" and an "act of hate" targeting a place of "solidarity and empowerment" for gays and lesbians. He urged Americans to decide whether this is the kind of "country we want to be."

Authorities said they had secured a van owned by the suspect outside the club. Meanwhile, a Swat truck and a bomb-disposal unit were on the scene of an address associated with Mateen in a residential neighbourhood of Fort Pierce, Florida, about 118 miles south-east of Orlando.

The attack follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, a YouTube sensation and former contestant on The Voice. She was killed after an Orlando concert by a 27-year-old man who later killed himself.

Pope Francis has expressed the "deepest feelings of horror and condemnation" over the massacre.

Vatican spokesman the Rev Federico Lombardi said the pontiff denounces the "homicidal folly and senseless hatred".

He added that Francis joins the families of victims and injured in "prayer and compassion".

Ronald Hopper of the FBI said Mateen was 29 years old, and an American citizen. He was not under surveillance at the time of the shooting.

Mr Hooper said Mateen had purchased at least two firearms within the last week or so.

Some 911 calls involving the shooter and the massacre have become federal evidence. He said the conversations involved Islamic State.

Mr Hooper said the suspected shooter had made inflammatory comments to co-workers in 2013, and had been interviewed twice. These interviews were described as "inconclusive".

In 2014, Mr Hooper added, officials found that Mateen had ties to an American suicide bomber. Mr Hooper described the contact as minimal, saying it did not constitute a threat at that time.